Yes, it's a full 24 hours, but a library doesn't use 24:00:00 to represent the last hour, it's 23:59:59. Once it hits 24:00, it rolls over to 00:00:00.
Hence my initial error of answering 23.
It's not valid, but I don't edit out erronous answers because I believe all data should be preserved, no matter how dumb it makes one look.
Due to the nature of Lemmy, nothing is ever truly deleted as it gets federated across multiple different instances.
It may eventually sync across most.
Deleting tends to only delete it on that instance, hence why when you reply to a deleted one, chances are you see it from your current instance's cache. Or vice versa.